Field of Invention
The present invention relates to a rare earth permanent magnet field, and more particularly to a high-performance NdFeB permanent magnet comprising a nitride phase and a production method thereof.
Description of Related Arts
The NdFeB rare earth permanent magnet is the widely applied basic electronic component and electrical apparatus element in the world, and is widely applied in computer, mobile phone, television, automobile, electrical machine, toy, sound system, automatic equipment, and magnetic resonance imaging. With the energy-saving and low-carbon economy requirements, the NdFeB rare earth permanent magnet is further applied in fields of energy-saving household appliance, hybrid electric vehicle, and wind power generation.
In 1983, the sintered NdFeB rare earth permanent magnet was firstly prepared through the powder metallurgy method by M. sgawaa et al., and the Nd2Fe14B phase and the grain boundary phase were confirmed to exist in the sintered NdFeB rare earth permanent magnet. The American patent publication, U.S. Pat. No. 5,645,651, granted in 1997, disclosed an R—Fe—Co—B metallographic structure. The emergence of the NdFeB rare earth permanent magnet represents the birth of the third-generation rare earth permanent magnet material. With the application of NdFeB, NdFeB is widely researched. Up to now, people are able to volume-produce the NdFeB rare earth permanent magnet having (BH)max of 52 MGOe; and it is found that: through replacing the light rare earth elements of Pr and Nd by the heavy rare earth elements of Dy, Tb and Ho, the coercive force of the magnet is increased from 12 kOe to 30 kOe, and the service temperature is increased from 80° C. to 180° C. With the application of the NdFeB rare earth permanent magnet in wind power generation, automobile, servo motors, energy-saving motors and electronic devices, the consumption of the heavy rare earth element, Dy, becomes more and more. Dy is a scarce heavy rare earth resource and few in the world, and now only produced from the ionic mineral in south China. A decrease of the consumption of Dy is important for protecting the scarce resource and decreasing the cost of the NdFeB rare earth permanent magnet.
In 1988, Chinese He Shuixiao et al. published an article in the magazine, Journal of Magnetic Materials and Devices, in China. He Shuixiao et al. found that: preparing powders through a fluidized-bed jet mill was able to obviously increase a magnetic performance of NdFeB. Thereafter, the fluidized-bed jet mill are popularized and applied in the field of NdFeB. The fluidized-bed jet mill has an obvious advantage that: during preparing the powders through the fluidized-bed jet mill, a portion of ultrafine powders is discharged with an airflow of a discharging pipe of a cyclone collector, wherein a discharging amount is 1-10% of a collecting amount. During the conventional process of preparing powders through the jet mill, because oxygen exists in the jet mill, a portion of ultrafine powders combines with the oxygen and forms oxides containing the rare earth; generally, the portion of ultrafine powders is discharged with the airflow of the discharging pipe of the cyclone collector and enters a filter. Because the ultrafine powders are inflammable, the portion of ultrafine powders is treated as wastes. American patent publications, U.S. Pat. Nos. 6,491,765 and 6,537,385 found that: during preparing powders through the jet mill, through removing part of the ultrafine powders having the particle size smaller than 1 μm, the magnetic performance of NdFeB was increased.
American patent publication, U.S. Pat. No. 6,468,365, and Chinese patent family application thereof, ZL99125012.5 disclosed an R-T-B sintered permanent magnet, wherein: O, C, N and Ca were listed as unavoidable impurities, and the impurities such as N were thought to affect the performance of the NdFeB sintered magnet. In 1990, Professor Yang Yingchang from Beijing University found SmFe12N has an excellent magnetic performance, and further found NdFe12N also has an excellent magnetic performance, wherein the Curie temperature of NdFe12N is higher than the Curie temperature of NdFeB by 200° C. Because NdFe12N is decomposed at temperature above 800° C., until now, it is still failed to produce the NdFe12N magnet, and only magnetic powders and the magnetic film thereof are able to be produced.
In order to increase the magnetic performance of the NdFeB rare earth permanent magnet material and meanwhile decrease the consumption of heavy rare earth materials such as Dy and Tb, Japanese enterprises have made a lot of researches. Japanese Shin-Etsu Chemical Co., Ltd. in Chinese patent publications, CN100520992C, CN100565719C, and CN101404195B, disclosed a high-performance R—Fe—B permanent magnet containing Dy, Tb, F and O. The average concentrations of F, Dy and Tb gradually increase from a center of the magnet to a surface of the magnet, and the distribution trends thereof are showed in FIG. 1. Moreover, rare earth oxyfluorides exist in the grain boundary of the grain boundary area which is from the surface of the magnet to an interior of the magnet at a certain depth. The permanent magnet was prepared through steps of: sintering the NdFeB magnet; adding oxides, fluorides or oxyfluoride powders containing Dy and Tb on the surface of the magnet; processing the magnet with a thermal treatment at a temperature lower than a sintering temperature in vacuo or an inert atmosphere; and absorbing Dy and Tb in the powders into the magnet. Through the above method, the coercive force of the sintered NdFeB permanent magnet is increased to a certain extent. However, according to the above method, the thermal treatment, which enables Dy and Tb to penetrate into the magnet, proceeds after sintering, causing the magnet becoming more crisp and harder, which brings troubles to subsequent machining and processing, leads to the easily broken edges and corners of the products during the transport process, and increases the rejection rate of the products.